April 10, 2003
09:00 AM (EDT)

News Release Number: STScI-2003-12

Far-Flung Supernovae Shed Light on Dark Universe

The full news release story:

Astronomers using the NASA Hubble Space Telescope's Advanced Camera
for Surveys (ACS) have found two supernovae that exploded so long
ago they provide new clues about the accelerating universe and its
mysterious "dark energy." The supernovae are approximately 5 and
8 billion light-years from Earth. The farther one exploded so long ago the universe may still
have been decelerating under its own gravity.

"We're trying to fill in a blank region where the universe's rate
of expansion switched from deceleration due to gravity to acceleration
due to the repulsive force of dark energy," says John Blakeslee, an
associate ACS research scientist at Johns Hopkins University,
Baltimore, Md., and lead author of a new paper due out in the June
Astrophysical Journal. "That's a real challenge, but the ACS is
making it very straightforward to find distant supernovae and get
detailed information about them."

"This beautifully demonstrates that the ACS is a 'supernova machine'
for probing the early universe," says co-investigator Holland Ford,
who headed the team that developed the ACS camera that was installed
on Hubble in March 2002. According to the Johns Hopkins astronomers,
the supernovae they discovered will be just the first of many to be
identified with the ACS.

Coupled with Hubble's powerful vision, the ACS can pick out the
faint glow of the distant supernovae. The ACS can then dissect their
light (by spectroscopy) to measure their distances, study how they
fade, and confirm that they are a special type of exploding star
that are reliable distance indicators.

In 2001 Hubble astronomers found a supernova even farther away. It
offered the first evidence the expanding universe was once
decelerating. Astronomers are using Hubble's new camera to go
supernova hunting for supporting evidence. "We have enough data on
the new supernovae to constrain both their distance and the amount
of dust obscuration," says Blakeslee. The filtering effects of
interstellar dust can lead to misinterpretation of the cosmic
distances unless carefully taken into account.

Type Ia supernovae are believed to be white dwarf stars that pull in
gas from an orbiting companion star. The white dwarf siphons off
mass until it hits a critical point where a thermonuclear "burning"
wave of oxygen, carbon, and heavier elements immolates the star in
a few seconds. The physics of the explosions is so similar from star
to star that all Type Ia supernovae glow at a predictable peak
brightness. This makes them reliable objects for calibrating vast
intergalactic distances.

The supernovae were found when ACS team members Daniel Magee
(University of California at Santa Cruz) and Zlatan Tsvetanov
(Johns Hopkins University) compared earlier Hubble images of the
same patch of sky with new ACS images and identified the two
supernovae. Follow-up observations were then conducted with ACS and
other Hubble instruments to get a detailed fix on their intensities
and distances from Earth.

Information from studies of Type Ia supernovae confronted
astronomers about five years ago with the stunning, unexpected
revelation that galaxies appeared to be moving away from each other
at an ever-increasing speed. They've attributed this accelerating
expansion to a mysterious factor known as dark energy that is
believed to permeate the universe.

Looking farther away into the universe (and, because of the distances
involved, further into the past), they've seen evidence that gravity
was at that time slowing the expansion of the universe. Astronomers
have very little data, though, on the period of transition between
these two phases, when the repulsion produced by dark energy began to
surpass the tug of gravity.

"Continued studies of supernovae will allow us to uncover the full
history of the universal expansion," Blakeslee says. "The sharper
images, wider viewing area, and keener sensitivity of ACS should allow
astronomers to discover roughly 10 times as many of these cosmic
beacons as was possible with Hubble's previous main imaging camera."